1. Field of the Invention
This invention relates to a zoom lens suitable for a camera for film, a video camera, a digital camera or the like, and particularly to a zoom lens which has achieved good correction of chromatic aberration and which is high in quality of image and compact as a whole.
2. Description of Related Art
In recent years, zoom lenses for a silver halide photographic camera, a video camera and a digital camera have been required to be high in variable power and high in quality of image due to the finer particles of photographic film and the higher pixels of CCD.
Also, in respect of the enlargement of a photographing area, irrespective of a silver halide photographic camera, a video camera or a digital camera, there is desired a photo-taking lens having a longer focal length and capable of effecting high magnification photographing and above all, the desire for a telephoto type zoom lens including a long focal length is great.
In a photo-taking lens of such a long focal length, however, the correction of curvature of image field and astigmatism is relatively easy, whereas it is very difficult to suppress the occurrence of chromatic aberration to a small level. Also, when the lens system is constituted by only an ordinary refracting optical system, to maintain satisfactory optical performance, the telephoto ratio of the entire lens system must be of a certain degree of magnitude or greater.
As a result, the lens system of such a long focal length becomes very great in the full length of the lens and becomes very inconvenient to handle. These difficulties becomes more remarkable when an attempt is made to overcome them by a zoom lens including such long focal length.
In Japanese Patent Application Laid-Open No. 4-301811, there is proposed a zoom lens which has, in succession from the object side, five lens units, i.e., a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power, a fourth lens unit of positive refractive power and a fifth lens unit of negative refractive power and in which the first lens unit and the second lens unit are moved to the object side and the image plane side, respectively, to thereby effect a focal length change from the wide angle end to the telephoto end, and the fourth lens unit is moved to thereby correct any fluctuation of the image plane resulting from the focal length change and the fourth lens unit is moved to thereby effect focusing.
Also, in Japanese Patent Application Laid-Open No. 4-301612 (corresponding U.S. Pat. No. 5,299,064), there is proposed a zoom lens of a five-unit type which has, in succession from the object side, five lens units, i.e., a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power, a fourth lens unit of positive refractive power and a fifth lens unit of negative refractive power and in which the second lens unit is moved to thereby effect a focal length change and the fourth lens unit is moved to thereby effect the correction of the fluctuation of the image plane resulting from the focal length change and focusing, and the whole of the lens system is approximated to a telephoto type to thereby achieve the shortening of the full length of the lens.
On the other hand, the recent zoom lenses have become such that because of the advance of the manufacturing technique and design application of an aspherical surface, the correction of various aberrations determining the performance of the lens is easy even in a construction wherein the number of lenses is small, and this has brought about a very great fruit in the downsizing and higher performance of the zoom lens.
Regarding chromatic aberration, however, the correction thereof depends on the color dispersing characteristic of a glass material forming the lens system and the achromatizing condition by the combination of positive and negative lenses and therefore, the correction of chromatic aberration by an aspherical surface effect cannot be much expected.
As methods of suppressing the occurrence and fluctuation of this chromatic aberration to a small level, in recent years, image pick-up optical systems using a diffractive optical element have been proposed, for example, in Japanese Patent Application Laid-Open No. 4-213421 (corresponding U.S. Pat. No. 5,044,706) and Japanese Patent Application Laid-Open No. 6-324262 (corresponding U.S. Pat. No. 5,790,321). These examples of the prior art are ones in which a diffractive optical element is applied to a single-focus lens system, and make mention of chromatic aberration, but bear no observation and description of the elimination or the like of the fluctuation due to chromatic aberration peculiar to a zoom lens due to zooming, and the application of a diffractive optical element to a zoom lens is not made.
A zoom lens utilizing a diffractive optical element is proposed, for example, in Japanese Patent Application Laid-Open No. 11-133305 (corresponding U.S. Pat. No. 5,978,153).
In this proposition, in a construction including a first lens unit of positive refractive power, a second lens unit of negative refractive power and a third lens unit of positive refractive power, a diffractive optical surface is disposed in the first lens unit to thereby correct chromatic aberration which is greatly aggravated by a focal length change. No mention is made of the fluctuation of aberrations by focusing, and in that point, there has been room for study.
Also, in U.S. Pat. No. 5,268,790, there is proposed a four-unit zoom lens comprising four lens units of positive, negative, positive and positive refractive power, respectively, in which a diffractive optical element is used in the second lens unit for focal length change or the third lens unit for correcting the fluctuation of the image plane resulting from a focal length change.
Generally in a zoom lens having high variable power and including a long focal length, the fluctuation of chromatic aberration due to a focal length change becomes great and it becomes very difficult to obtain high optical performance over the entire range of focal length change and over the whole of the object distance.
Particularly in a zoom lens of a high variable power ratio in which the zoom ratio is about four times, it is often the case that a cemented lens is used to correct chromatic aberration occurring in each lens unit. For the lens units, there is adopted a method of using an aspherical surface to thereby curtail the number of lenses in the lens units and shorten the full length of the lens.
However, if the number of lenses is decreased, the element for correcting chromatic aberration will become insufficient and it will become difficult to well correct the fluctuation of chromatic aberration resulting from a focal length change.